From German Published Patent Application Nos. 1,934,668 and 1,944,825 there is known a dosing device for an instrument used for carrying out chemical analyses. This dosing device contains an aspirator tube for aspirating a liquid from a sample vessel and successively metering the aspirated liquid into different reaction vessels. The aspirator tube is in connection with a rinsing liquid pump. Following each aspirating and metering operation which is effected by means of a metering pump also connected to the aspirator tube, residual sample liquid is displaced from the aspirator tube by rinsing liquid which is forced into the aspirator tube by the rinsing liquid pump. As a result, the aspirator tube is completely filled with rinsing liquid at the start of each operating cycle.
U.S. Pat. No. 4,111,051 is concerned with an apparatus for dosing sample liquid taken from sample vessels into a graphite furnace for electrothermally atomizing the sample liquid in an atomic absorption spectrophotometer. This apparatus contains an aspirator tube which is connected with a metering pump as well as a rinsing liquid reservoir via a rinsing liquid pump. At the start of each operating cycle, the aspirator tube is filled with rinsing liquid. The aspirator tube is introduced into a sample vessel. A predetermined amount of rinsing liquid is aspirated from the aspirator tube by means of the metering pump and, correspondingly, a predetermined amount of sample liquid is taken up from the sample vessel. Subsequently, the aspirator tube is introduced Into the graphite furnace. The metering pump forces the sample liquid out from the aspirator tube into the graphite furnace. Thereafter, a rinsing operation is effected. For this purpose, the aspirator tube is introduced into a rinsing vessel constituting an overflow vessel. Therein the aspirator tube portions which had contacted the sample liquid, are immersed into rinsing liquid. The rinsing liquid pump delivers a preselected volume of rinsing liquid via the aspirator tube into the rinsing vessel and via the rinsing vessel into a waste container. During this operation, the aspirator tube is internally and externally rinsed. In this manner there is prevented carry-over of sample liquid when the aspirator tube is subsequently immersed into a further sample vessel for aspirating and metering the next-following sample liquid.
A similar arrangement is described in U.S. Pat. No. 4,068,529.
There are also known automatic sampler dispensers in which the aspirator tube is vertically movably guided at a carrier and can be vertically reciprocated by means of a stepping motor. The carrier is controlled for movement in a horizontal plane such as to be successively placed above different sample vessels and the inlet of an analytical instrument like, for example, a high-pressure liquid chromatograph.
In these known automatic sample dispensers the aspirator tube is immersed into the sample vessel down to a predetermined depth in each case. During standard operations, the aspirator tube is introduced down to the lowest location of the sample vessel in order to ensure immersion of the aspirator tube into the sample liquid. The aspirator tube thus is wetted by the sample liquid also externally along a relatively great length. This may result in considerable measuring errors in highly sensitive measurements such as, for example, atomic absorption spectroscopy due to sample carry-over. Such undesirable effect may occur when successively analyzing samples containing extremely high and extremely low concentrations of the element to be determined. This problem can be reduced by using the aforedescribed rinsing operation. However, such rinsing operation requires an additional period of time. Furthermore, sample carry-over may also be effected in the rinsing vessel.
Therefore, there are known means for measuring and regulating the immersion depth of a dosing tube or pipette into a liquid.
It is known to pneumatically detect the liquid surface. In such apparatus, a level sensor has an air exit opening which is restricted when the level sensor approaches the liquid surface. There is thus caused a pressure increase which serves as a control signal. Such pneumatic level sensors are described, for example, in European Published Patent Application No. 273,128 and German Patent No. 3,039,475.
It is further known to set a pipette or dosing tube into mechanical oscillations. A sensor taps the oscillation amplitudes which serve as the control signal. These oscillation amplitudes decrease once the pipette or dosing tube is immersed into the liquid because, then, the oscillations are attenuated. An example of such level sensor is described in German Patent No. 3,614,961.
Furthermore, there are known in combination with pipettes or the like, level sensors which respond to the electrical conductivity of the liquid. Examples of such arrangements are described in German Patent No. 3,219,251 and German Published Patent Application Nos. 3,839,896; 3,905,622 and 3,909,515.
In an arrangement as described, for example, in U.S. Pat. No. 4,736,638, a turntable contains an electrically conductive plate at which the sample vessels are arranged. A low frequency oscillator is connected to the plate and generates an electrical field. A detector which is connected with the dosing tube, responds to this electrical field. When the dosing tube arrives at the liquid surface, an output signal is generated and interrupts the downward movement of the dosing tube.
Also, optical level sensors are known in combination with dosing tubes or the like for detecting the surface of liquids.
A dosing tube and a detector connected therewith are described in, for example, European Published Patent Application No. 250,671. The detector contains a light transmitter and a light receiver. Optical imaging means produce a substantially V-shaped path of rays. When the image of the light transmitter is located at the surface of the liquid, reflected light impinges upon the light receiver and generates a signal.
German Patent No. 3,113,248 describes an apparatus for taking up liquids from vessels at an analytical scale by means of a dosing tube which is immersed from above into the liquid to be taken up. A bundle of optical fibers is attached to the dosing tube and serves as level sensor. The optical fibers end above the lower end of the dosing tube by an amount corresponding to the immersion depth. A first number of the optical fibers is in communication with a light transmitter and a second number of the optical fibers is in communication with a light receiver.
In a further arrangement such as shown, for example, in German Patent No. 3,149,211, a predetermined immersion depth of the dosing tube is signalled by means of a light barrier extending transverse through the sample vessel.
German Published Patent Application No. 3,619,870 relates to a reflectometer for investigating glass fibers and containing a light transmitter, a coupling fiber and a photoelectric receiver. The light which is transmitted by the light transmitter, enters the coupling fiber through the end face thereof. The reflected light is returned by the coupling fiber and is coupled out from the coupling fiber by means of a semipermeable reflector for reflection to the photoelectric receiver.